For example, as for a technique of a USB device, there is a USB interface as a communication interface between a personal computer and a peripheral device thereof. There are a keyboard, a USB memory, and the like as a USB-compliant peripheral device. These devices generally do not have a power supply inside and are supplied with power by such a host as a personal computer. In the case of devices having only some specific function like a keyboard, since power consumption required for internal operation thereof is small, there is no problem in using such a device to PDAs or mobile phones which have a battery of small capacity.
As to a USB device having many functions, however, since power consumption required for its operation becomes large, it is difficult to use such a device for PDAs or mobile phones from the aspect of capacity of battery.
Consequently, as a technique for reducing power consumption required for operation, there is described a technique of shutting off power supply to an unnecessary block during connection to a USB by a switch in U.S. Patent Application Publication No. 2005/0050371 (corresponding to Japanese Patent Application Laid-Open Publication No. 2005-71274). In Japanese Patent Application Laid-Open Publication No. 2004-341699, there is described a technique of causing a user to select a usage function when a host rejects connection with a high power device to change settings so as to make it a low power device. In Japanese Patent Application Laid-Open Publication No. 2004-86359, there is described a technique of determining an operation mode and power consumption of a device according to negotiation between a host and a device at a start time.